Determining the temperature of a solid object is typically accomplished by measuring the temperature of its surface. While it is preferable, for reliability of the measurement, to position a temperature sensor within the object itself so that the sensor is immersed in the material to be measured, this is often not a practical solution.
Contact thermometers, such as thermocouples, thermistors and RTDs (resistance temperature detectors), are often employed to measure the temperature of a surface. Achieving adequate thermal contact between the contact thermometer and the sample surface to be measured can be very difficult. Contact-type temperature sensors suffer from the universal drawback that the very act of contact with the sample drains heat from, or adds heat to, the sample, due to the thermal mass of the sensor relative to the object. Furthermore, in the case of contact with a solid surface, as opposed to immersion in a fluid, true contact is achievable only at a few points and hence the sensor never reaches the exact temperature of the sample surface being measured. As a result, the temperature read by the contact thermometer can deviate substantially from that of the sample.
The problems with contact thermometers are exemplified under low ambient pressure conditions, where conduction by gas in the gap between the sample surface and the thermometer is diminished until it becomes zero in a vacuum. Under these conditions, thermal contact is worsened due to the lack of air molecules present to help conduct heat from the surface to the sensor. Strong radio frequency or microwave fields are also known to negatively affect the ability of these contact thermometers to accurately measure surface temperatures. As such, all contact thermometers which are not incorporated into the surface itself suffer either because good thermal contact is extremely difficult to achieve or because they drain excessive heat from the surface to be measured.
One solution to the contact-type of temperature thermometer is to utilize a thermometer that does not require physical contact with the surface to be measured. One such thermometer known in the art utilizes infrared radiometry. With this approach, thermally-generated infrared radiation from the surface is collected and focused onto an infrared detector. While this is the most common non-contact thermometry technique used in the current state of the art, it is highly unreliable unless the emissive properties of the surface are exceedingly well characterized. Other problems include the need for line of sight access to the surface, poor sensitivity at moderate to low temperatures and the inability to measure transparent materials utilizing infrared radiation.
Accordingly, reliable measurements of surface temperatures have proven to be difficult. Measurement techniques currently known in the art having the highest levels of reliability are typically tedious and time consuming to perform.
Over the past two decades, the fluorescence-decay rate (FDR) type fiber optic temperature sensor has emerged as an increasingly important class of thermometric devices. The response of these sensors derives from the temperature dependent decay time of some optically induced fluorescence produced by a phosphor which is either attached to one end of the optical fiber or coated on the surface of the object being monitored. FDR sensors are generally recognized as the most accurate optically based systems for temperature measurement in the art. As a result, in spite of their relatively high cost, they are in routine use now in industries as diverse as semiconductor manufacturing, power generation, and metal forming when electromagnetic interference from the environment prevents the use of electrically based sensors or when moving materials are to be interrogated.
U.S. Pat. No. 7,104,683, issued on Sep. 12, 2006, incorporated herein by reference, describes a thermally compensated fluorescence decay rate temperature sensor. It requires for its operation at least four temperature measurements, made at two positions, with the phosphor tip at two different heating laser power levels. One pair of measurements should be made with the probe in contact or near contact (less than 1 mm) with the surface, and the second pair slightly further away (less than 3 mm). This in practice may cause some difficulty in certain situations.
Accordingly, what is needed in the art is a surface temperature measurement technique which is both fast and convenient, while at the same time, being reasonably accurate, reliable and adaptable to many environments.